1. FIELD OF THE INVENTION
This invention relates to the field of programmable clocks to be used in demodulating receiving channels.
2. BACKGROUND ART
With the increased popularity of computers and data processing systems, the need for communication between computers and data bases has increased. A popular tool for implementing communication between computers is the modem, which allows communication over telephone lines of data in serial form. Modem is short for "modulator/demodulator".
Raw data signals, also known as "base band" signals, are inappropriate for transmission at high speed. In order to efficiently transmit the data over, for example, phone lines, it is first modulated to a higher frequency by using the data signal itself to modulate a carrier wave. For example, phase shift key (PSK) and quadrature amplitude modulation (QAM) schemes are typically used as a way of transmitting data at high frequencies.
Correspondingly, when the modulated data is received by another modem, it must be demodulated, that is, the carrier must be removed, leaving the data signal.
When data is received by a modem, it is filtered and amplified as necessary prior to being sampled and converted to a digital signal. There is a certain amount of noise associated with phone line transmission. This noise, although not necessarily at the sampling frequency, may cause aliases to appear, resulting in data compromise or loss. In order to eliminate the noise, anti-alias filters are utilized.
In the prior art, anti-aliasing filters have typically been implemented through continuous time, resistor-capacitor (R-C) circuitry. However, such R-C anti-alias filters have a poor time constant tolerance which may be as large as plus or minus 50%. In addition, a large silicon area is required when implementing R-C filters in integrated circuit format.
In order to facilitate anti-aliasing, it is desired to sample data with a high frequency clock. However, subsequent switched-capacitor stages require low sampling rates to minimize silicon area and to reduce the processing rate required in the following demodulating stages.
In the prior art, the low frequency sampling signals were provided by programmable timers on digital signal processors or with costly external logic. Therefore, it is desired to provide a low cost independent programmable timer associated with anti-alias filters and the gain stage of modem receivers.
It is another object of the present invention to provide an accurate anti-aliasing filter with a close tolerance time constant.
It is yet another object of the present invention to provide an anti-alias filter which consumes very little silicon area in integrated circuit applications.